1. Field of the Invention
The present invention is directed generally to lithography. More particularly, the present invention relates to maskless lithography.
2. Related Art
Lithography is a process used to create features on the surface of substrates. Such substrates can include those used in the manufacture of flat panel displays (e.g., liquid crystal displays), circuit boards, various integrated circuits, and the like. A frequently used substrate for such applications is a semiconductor wafer or glass substrate. While this description is written in terms of a semiconductor wafer for illustrative purposes, one skilled in the art would recognize that this description also applies to other types of substrates known to those skilled in the art.
During lithography, a wafer, which is disposed on a wafer stage, is exposed to an image projected onto the surface of the wafer by exposure optics located within a lithography apparatus. While exposure optics are used in the case of photolithography, a different type of exposure apparatus can be used depending on the particular application. For example, x-ray, ion, electron, or photon lithography each can require a different exposure apparatus, as is known to those skilled in the art. The particular example of photolithography is discussed here for illustrative purposes only.
The projected image produces changes in the characteristics of a layer, for example photoresist, deposited on the surface of the wafer. These changes correspond to the features projected onto the wafer during exposure. Subsequent to exposure, the layer can be etched to produce a patterned layer. The pattern corresponds to those features projected onto the wafer during exposure. This patterned layer is then used to remove or further process exposed portions of underlying structural layers within the wafer, such as conductive, semiconductive, or insulative layers. This process is then repeated, together with other steps, until the desired features have been formed on the surface, or in various layers, of the wafer.
Step-and-scan technology works in conjunction with a projection optics system that has a narrow imaging slot. Rather than expose the entire wafer at one time, individual fields are scanned onto the wafer one at a time. This is accomplished by moving the wafer and reticle simultaneously such that the imaging slot is moved across the field during the scan. The wafer stage must then be asynchronously stepped between field exposures to allow multiple copies of the reticle pattern to be exposed over the wafer surface. In this manner, the quality of the image projected onto the wafer is maximized.
Conventional lithographic systems and methods form images on a semiconductor wafer. The system typically has a lithographic chamber that is designed to contain an apparatus that performs the process of image formation on the semiconductor wafer. The chamber can be designed to have different gas mixtures and grades of vacuum depending on the wavelength of light being used. A reticle is positioned inside the chamber. A beam of light is passed from an illumination source (located outside the system) through an optical system, an image outline on the reticle, and a second optical system before interacting with a semiconductor wafer.
A plurality of reticles are required to fabricate a device on the substrate. These reticles are becoming increasingly costly and time consuming to manufacture due to the feature sizes and the exacting tolerances required for small feature sizes. Also, a reticle can only be used for a certain period of time before being worn out. Further costs are routinely incurred if a reticle is not within a certain tolerance or when the reticle is damaged. Thus, the manufacture of wafers using reticles is becoming increasingly, and possibly prohibitively expensive.
In order to overcome these drawbacks, maskless (e.g., direct write, digital, etc.) lithography systems have been developed. The maskless system replaces a reticle with a spatial light modulator (SLM) (e.g., a digital micromirror device (DMD), a liquid crystal display (LCD), or the like). The SLM includes an array of active areas (e.g., mirrors or transmissive areas) that are individually controlled to form a desired pattern. These active areas are also known in the art as “pixels.” A predetermined and previously stored algorithm based on a desired exposure pattern is used to control the pixels. Each pixel in an SLM can vary its optical properties (e.g., amplitude/phase transmittance) in a controllable manner so as to provide a variation of a dose delivered to the wafer surface.
In a typical embodiment, each pixel can assume any of a limited number of discrete states, each corresponding to a certain level of dose gray-scaling. One of the many states that the pixel can assume corresponds to the pixel sending no light to the exposure area. This state may be referred to as the dark state or the OFF state. Other states of the pixel correspond to the pixel being modulated so that it sends a certain fraction of the incident light to the exposure area. In order to be able to control the printed pattern (e.g., a position or width of a printed line), it is desirable to have as many grayscale levels as possible. However, the number of grayscale levels achievable by increasing the number of discrete pixel states is limited due to at least the following reasons.
A pattern on an SLM typically has to be updated for every laser pulse if the wafer scan is continuous with exposures occurring during the short laser pulses. If exposures are performed with a continuous light source, but the wafer is either at rest during the exposure or the smearing of the exposure is compensated, the pattern has to be updated at least very frequently. As a result, a high data transfer rate to the SLM has to be maintained. This data transfer rate increases proportionally to the logarithm of the number of discrete states, and the limitation on the maximum possible data transfer rate results in a limitation on the number of pixel states and number of grayscale levels.
Also, having a larger number of pixel states makes both the design of an SLM and the control over the states more difficult.
Therefore, what is needed is a maskless lithography system and method that would allow achieving a larger number of grayscale levels without increasing the number of distinct pixel states.